Signaling-circuit.



G. A. CAMPBELL.-

SIGNALING CIRCUIT.

APPLlcATloN HLED AuG.1a.1917.

Patented J an. 22, 1918.

3 SHEETS-SHEET I.

IIL

y I N V EN TOR. M Campbell BY .h

A TTORNEY `G. A. CAMPBELL.

SIGNALING CIRCUIT.

APPLICATION FILED AuG.IY8. 1911.

Patent@ m1221918.

3 SHEETS-SHEET 2.

12 I INVENToR. Z We A TTORNEY'.'

G. A. CAMPBELL.

SI'GNALING CIRCUIT.

APPLICATION FILED Aucme. Ism.

Patented Jan. 22, 1918.

3 SHEETS-SHEET 3- INI/EN TOR. WUI/.Cm Mz ATTORNEY line noise.

UNITED STATES PATENT OFFICE.

GEORGE A. CAMPBELL, 0F MONTCLAIR, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION 0F NEW YORK.

smNALING-CIRCUIT.

Specification of Letters Patent.

I To all whom t may concern:

Be it known that I, GEORGE A. CAMPBELL, residing at Montclair, in the county of Essex and State of New Jersey, have invented certain Improvements in Signaling-Circuits, of which the following is a specification.

This invention relates to circuit arrangements for signaling systems wherein signals may be either transmitted from or received at the same station. In its more specific aspects this invention is embodied in a subscribers telephone station, hereinafter termed, in accordance with common usage, a substation, and more particularly in the combination of a substation and a telephone line. Its object is to provide a signaling circuit arrangement which kin cooperative combination with a similar' and equal communicating arrangement or station shall deliver the maximum amount of energy to the receiving apparatus of said communicating station or arrangement. A further object is to provide an arrangement such that the receiving apparatus is protected from interference by the transmission energy originating at the same station. In other words its object is to provide signaling means characterized by the maximum possible ratio of received to transmitted energy and further characterized by the absence of side tone.

The object of the invention is attained, in its specific aspect, by providing a substation consisting of transmitter, receiver, auxiliary resistance, and a transformer having a plurality of windings which, in combination with a telephone line, shall satisfy the following fundamental requirement: Given two identical substations designed for invariable twoway communication, and connected by a line of given impedance and length, the amount of energy absorbed by the receiver at the receiving station shall be the maximum part of the total telephonie energy developed by the transmitter at the transmitting station consistent with invariable twoway communication, and, as hereinafter explained, consistent with a desirable amount of discrimination against disturbing This fundamental requirement may be stated in terms of the following subordinate requirements which are necessary for its satisfaction: (l) The transmitter and receiver shall be conjugate, that is, there shall be negligible side tone in the receiver in consequence of the actuation of the transmitter by sound waves; (2) the line and auxiliary resistance shall be conjugate in order that none of the energy absorbed by the substation from the `line shall be wasted in said auxiliary resistance; (3) for a given line having a definite impedance the telephonic energy delivered by the transmitter shall be a maximum; (t) the amount of energy delivered by the line to the substation shall be a maximum, in other words the impedance of the substation as seen from the lines shall be equal to the impedance of theline; (5) at a small sacrifice of efficiency it shall be possible to discriminate effectively against disturbing line noise as distinguished from the telephonie signals from the communicating station.

A substation satisfying the above-mentioned requirements is ideal in that its overall efliciency from transmitter of one substation to receiver of the communicating substation is a theoretical maximum which cannot be exceeded by any invariable substations whether satisfying the requirement of transmitter and receiver conjugacy or not. It is further ideal in the sense that a minimum number of elements is employed since at least one auxiliary element is necessary to secure freedom from side tone.

It might be inferred that the addition of an auxiliary resistance element, necessary as it is to secure freedom from side tone, would at the same time necessarily reduce the efficiency of the substation since energy is unayoidably wasted in said auxiliary resistance. That this is not the case and that the eiiiciency of the substation of my invention is a theoretical maximum which cannot be exceeded by any two-way substation whether with or without side tone, the following considerations will show. The simplest form of substation for invariable twoway communication is that in which the receiver and transmitter are connected in series with each other across the line. In such an arrangement the over-all eiiiciency is a maximum when the resistance of the receiver is equal to that of the transmitter. When this condition is satisfied obviously fifty per cent. of the energy delivered by the Patented Jan. 22, 1918.

liney to the substation is wasted in the transmitteiand fifty percent. of the energy delivered by -the'transmitter is wasted 1n .the receiver. Further, such an arrangement labors under the disadvantage of full s ide In the vrsubstation of my "invention,

tone.

- fifty per cent. of the energy delivered by 4 The transmitting eiiciency is therefore also the line to the substation is 'wasted in the transmitter but none in the auxiliary resistance if said auxiliary resistance and said line are conjugate; hence the eiiciency Y is as great as that ofthe. simple series substation. ,When transmittin no energy is wasted in the receiver but fi ty percent.

of vthe energy delivered Vby `the transmitter 'is wasted in the auxiliary resistance.

a theoretical maximum and neither trans- "mitting norV receiving eiiiciency is 're- `duced by the-addition of the auxiliary resistance which is necessary to securefreedom from sidetone. YThe foregoing considerationwill serve to explain the desirability of having the line and auxiliary resistance l conjugate as well as the transmitter'and receiver. Y Y

'nV In my presentinvention I provide a'substation comprising transmitter, receiver,

auxiliary resistance and transformer and'so Y proportion said component elements and so relate them to a'telephone line and to each other that,y in combination with said telephone line, said substation satisfies all of the foregoing requirements. 5

Y,I have discovered that the above-inentioned requirements may be satisfied by a large number Yof arrangements employing the minimum number of elements and all equally eficient and without side tone. While theoretically all 'these arrangements are v drawing in which:

equally good, practical considerations make certain arrangements preferable.`

My invention will now vbe fully understood by reference to the accompanying Figures 1,5, 9, 13, 17 and 21 are schematic diagrams of six forms of substations in accordance with .this invention, said diagrams vbeing so arranged `as to clearly indicate the equivalence ofthe different circuits." Figs 2, 6, 10, 14, 18 andV 22 are circuit diagrams showingithe arrangement ofthe several forms of substations in greater detail. Figs.

3, 7,11, 15, 19 and`23 arecircuit diagrams indicating the relative direction vduring transmission 'of current flow in the several circuits rshown in Figs.2, 6, 10, 14, 18 and 22, respectively, the Ydotted lines indicating Y VVelements through which nol current flows. Figs. 4, 8, 12, '16, 20 and 24 are similar diagrams indicating-theV relative.direction of current flow during reception. In the above diagrams the direction of winding of the transformer coils and the direction of current flowV as indicated are merely conventional and Vonlyhold true'A for particular i specification.

1,254,i ie

values of therelements ofV the substation. It will beclear from an inspection of the design formulae hereinafter set forth that the winding ratios rand y. may, in genera-l, Y

eral theoretical discussion will now be viven which applies to all substations satisfying the'requirements heretofore stated in this 'In this discussion andthe equations yand formulae included in this specification the subscripts 1, 2, 3 and 4 Will refer to transmitter, receiver, auxiliar sistance and-line respectively. -Thus 1, I2,

I3, I,l will denote the currents flowing in transmitter, receiver, auxiliary resistance yand line'respectively, while R1 will denote the resistance of the transmitter, R2 the resistance of the receiver, etc.

Consider-a substation consisting of trans- 'mitten receiver, auxiliary resistance and appropriate transformer windings, connectedV to a line of given impedance.

Y In practice the line connects two similar and equal substations between which communication is established. It is a well known principle that if a terminal impedance isl connected to a source of electroinotive force through a line of impedance Z=R{-z'R4,l Y where R4 is the resistance and Rf, the reactance component of the impedance, the

terminal impedance must be R-z'Ri for ,maximum absorption of energy. In particular, if the line impedance has no Vreactancecomponent, the impedance ofthe terminal arrangement .as seen from the line should be equal to the resistance component of the impedance of the line. The condition, then, thatfthe substation shall have maximum energy absorption from the line ris that its impedance, as seen from the line, shall be equal to the lineim edance.

The significance of the oregoing statement may be explained by reference to Fig. 2, as'. follows: Let the substation be disconnected from the line and `let the impedance of the substation be measuredV across terminals afand'a. Then the inipedanceso measured shall be equal to the impedance of the line. With the line terminated at each endby a substation satisfying this conditionthe line may be replaced, as regards transmission from eithersubstatiomfby an impedance element of resistance equal to the impedance of the line. Any reactance effect, which is in practice small,l may be eliminated by neutralizing reactance and, therefore, need not be considered. The condition, then, that the substation have maximum energy absorption from the line is that its impedance as seen from the line be a pure resistance of value equal to the impedance of the line. This condition is evidently equivalent to the following requirement; let an electromotive force be impressed on the substation terminals through a resistance equal to the impedance of the line; then the energy consumed in the substation shall be equal to the energy consumed in said resistance.

Further, line and auxiliary resistance are conjugate by requirement (2), as hereinbefore stated, or in other words, the auxiliary resistance is connected to points of equal potential with respect to an electromotive force applied to the line terminals. Moreover, the impedance of the substations as seen from the line should be equal to that of the line. Let, then, an electromotive force E,L be impressed through a resistance R4 on a substation whose transmitter and receiver resistances are R1 and R2 respectively, and let the resultant currents in line, transmitter and receiver be I4, l1 and I2 respectively, the impedance across the substation terminals must be R4 as seen from the line and the total resistance in series with E is 2K4, and since the current in the line is L, it follows that The total energy consumed may then be expressed by the formula Since the energy consumed by the substation is equal to that consumed by the resistance R4 and is therefore one-half of the total energy consumed, it follows that requirement t may be formulated by the following:

ign, ign1 ign, 14n

ance. Then. for maximum Output, it follows that i zni 2n i 2n (Ef 3) .-4R (2) Equation (2) is the analogue of equation (l) and may be interpreted as follows by reference to Fig. 2. Let the transmitter be disconnected from the terminals Z) and e and let the impedance be measured across said terminals. Then if equation (2) is Satisfied the impedance so measured is equal to the impedance of the transmitter. In other words, the impedance of the combination as seen from the transmitter is equal to 'that' of the transmitter itself.

As hereinafter shown for the particular embodiments of my invention, equation (2) follows as a consequence of the conditions of double conjugacy and equation (l). Therefore the foregoing four requirements impose but three restrictions on the substation.

To complete the general discussion it remains to consider the energy division between receiver and transmitter when receiving, and between line and auxiliary resistance when transmitting. Let *W0 be the total amount of telephonie energy developed by the transmitter at the transmitting substation; then, by equation (2), l/QVO is the amount of energy delivered to line and auxiliary resistance. Let the amount of energy taken by the auxiliary resistance be a; times that taken by the line, then the amount of energy taken by the lineis l Umm) so that the transmitting etlicieucy is measured by Of the total energy delivered to the receiving substation, let the transmitter absorb y times that absorbed' by receiver; then the receiving eiiiciency is measured by The over-all efciency from transmitter of one station to receiver of communicating station ie clearly proportional to the product of the transmission efficiency and receiving ethciency; therefore the over-all efficiency is by formulae and (il):

() (rift) if m and ',1/ were independent7 clearly the over-all efficiency would be a maximum for aezyzt). For all substations embodying the principles of my invention it may be readily shown, however, that and y are connected 40. follows that:

by the relation :12g/:1. Eliminating as from Y the above formula b means of this relation,

the expression for t e over-all efficiency becomes Y (i+1/r (5)' I, Inorder to demonstrate the above statement, namely that yzl, designate the `ele- V ments or branches T, R, X and Lby 1, 2,

3j and 4, respectively, and let 1 and 2 be con-V l jugate and also Sand 4 be conjugate. Y Further it will be `assumed for an electromotive force in branch 4, equation v(1) is satisfied i. while for an electromotive force in branch (l), equation (2) is satisfied. Let S11 denotethe currentproduced in branch or element 1 by a vunit electromotive force in branch-1,` S11 the current produced inY branch Y 2 by a unit electromotive force in branch 1, etc. Then by the conjugacy of branches l and 2 and branches 3 and 4, it follows that y n S12=S34=0 y Also by equation (1) (S44) 2R4 (S42) 2B@ 'l' (S41) ZRi l) andV by equation (2) Y Y' (S11)2R1 2Ra "l" (SulzR/l. 4Rl (2 l) Now it is 'a fundamental principal which is deducible from elementary algebra, that S11=S11. That is, the'current set up in branch 4 by a unit electromotive` force in branch 4 is equal to the current set up in branch 4 by an unit electromotive force in branch 1. Multiplyingequation (1') byA R,1 and equation (2)V by and subtracting it Now' in accordancel with the notation adopted in this specification, the energy con- 45 sumed in branch Vv3 is a: times that consumed in branch 4 when an electromotive force acts inbranch 1; therefore 1 Also theenergy consumed in branch 1 is y times that consumed Vin branch 2 when an "electromotive force acts 1n branch 4; whence (S14)2I1:1:l/(S2)2R'2. 55 Multiplying. (b) and (o) f y.

(SiaeRiRFa/(Sarn.k (a From- (a) 'and (d) it follows at once that w1 :1. 1 g Y 80 'Obviously the expression given by formula (5) is ajmaximum when 3];1'. This vmeans that for aY given amount of telephonie energy developed in the transmitter at the transmittings'ubstation a maximum Vamount @.65 is` usefully delivered to the receiver at Vthe Y Y with a small loss'in over-all efficiency.

receivingsubstation connected by the line, when y=1. Since the maximum amount of l energy in the `receiver is the prime desideratum of telephony, it would appear that the substation'should be designed to make g/:L 70 Another consideration, however, modifies this conclusion somewhat, namely, the effect of line noise. Since lthe line noise originates in the line the amount delivered'to the receiver isproportional` to 1 1 -ly (see equation 4) while the amount of energy delivered from the transmitter of thecommunicating station is proportional to L (1 +w- (see equation 5). The ratio of thev latter 85' to the former is Y y l Y Y 1+',l/ and lthis increases as y increases beyond unity. It will'be clear., then, that if y `is made greater than unity the substation discriminates against line noise as compared with the signalsit is Vdesired to receive. The amount of discrimination desired depends of course on the amount of line noise present. For conditions occurring in practice I have found by experimentthat a desirable value for y is 1.4. With this value of y'the over-all-eiliciency is reduced 2.8% 10o Vbelow the maximum for yzl, while the receivin'geiliciency alone is reduced 16.6%.

It is thus seen that a good degree of discrimination against vline noise is obtained The above considerations as to over-all eiiiciency 'and discrimination against line noisemay be formulated as R1V(I1) zzyRz (I2) 2 (6) for an electromotive force inserted in the Y vThe auxiliary resistance X is connected in seriesk with the winding N3 between termi- 125 nals a and efand the line L is connected one lside tothe terminal a and the other side tor-the junction Vpoint d between the auxiliary'resistance X andthe winding N3. If

desired a condenser may beV insertedV series with vthe winding N3 to prevent the flow of direct current through said winding. Direct c urrent from a central energy source is supplied to the line L and flows .in parallel through the transmitter and receiver and thence through the auxiliary resistance X.

The operation of the substation of Fig. 2 in transmitting will be clear from an inspection of Fig. 8 in which the arrows indicate the relative directions of current flow. The actuation of the transmitter varies the current flowing in the transmitter circuit, its action being equivalent to setting up in the transmitter a variable electromotive force which causes an alternating current to flow. The direction of this current at a given instant of time is indicated by the arrow designated Il. Since the transmitter and receiver are conjugate during transmission, no current will flow through the receiver and the auxiliary resistance vX is Vin a series circuit with the transmitter. Consequently the current I3 flowing through the auxiliary resistance is equal tothe current I1 flowing through the transmitter. At the terminal a. the current I divides and a current Il,c flows over the line and back to the junction point (l, while a current I3--I L flo-ws through the winding N3 to the point d. Potentials are induced in the windings of such value and direction as to reduce the terminal c to the same potential as the terminal c, so that no current flows through the receiver during transmission.

They action when receiving is indicated in Fig. t. A potential applied to the line causes a current I4 to flow to terminal a, through the winding N3 to the terminal d and back to the line. Since during reception the line and the auxiliary resistance X are conjugate, no current flows through the auxiliary resistance. The current flowing through the winding N3 sets up electromotive forces in the windings N2 and N1, whereby a current I, `equal to I2 flows in a local circuit from terminal a, throughv the winding N1, transmitter T, receiver R and winding N2 back to terminal a.. The electromotive forces induced in these windings reduce the terminal e to the same potential as the terminal d, so that no current flows through the auxiliary resistance. As a result no energy loss occurs in the auxiliary resistance and while the lreceived current flows through the transmitter and receiver `in series, the energy thus wasted is no greater than the transmitter loss'in a standard substation.

The proportioning of the arrangement of Fig. 2 to satisfy the fundamental requirements of the substationiof my invention will now be given. In deriving the design formulae, the resistances of the transformer windings will be ignored. It will be assumed further that there isno magnetic leakage between the transformer windingsand that the self impedances are very large compared with the impedances ofthe component elements of the substation or the impedance of the line. Experience has shown that these simplifying assumptions are justilied and that the assumed conditions may be closely realized in practice by careful design.

To formulate the condition, for conjugacy of transmitter T and receiver R, assume an electromotive force in the transmitter cir` cuit and assume that the desired condition of conjugacy is satisfied. In other words, assume that the terminals of the receiver are at points of equal potential with respect to an electromotive forcek applied to the transmitter. Let Il, I2, I3 and I4 denote the currents flowing in the transmitter, receiver, auxiliary resistance and line, respectively, and R1, R2, R3 and R, denote thel resistances of the corresponding elements. Furthermore, let the number of turns of the transformer windings connecting terminals a` -and b, a and c and a and (Z, respectively, be represented as follows:

The condition of conjugacy of the re ceiver with respect to the transmitter requires that no current flow through the receiver and hence I2=0. It is also clear from Fig. 3 that if no current Hows through the receiver, I3=I,. Assuming a potential Va, applied across the terminals a and e, since the potential so applied must'equa'lthefalgebraio sum of the potential drop through the winding N3 and the IR drop through the auxiliary resistance X, we have vaga-313mm,

Where K designates the potential drop per turn of the transformer windings.

Since terminals c a'nd? are at the same potential, it follows that j and i through the winding Nl'will equal the 1R Y Il'urtherm'ore Vsince'the algebraic sum of the ampere turns ofthe three windings of the .transformer must equal zero:

nJmUs-L) 1,-. n, 7h-n1 Y Combining the above equations, we get I n, Y Tian-1, (7)

R i RFmm J (8) When receiving (see Fig. 4) the lineland auxiliary resistance are conjugate sothat no current iiows through the auxiliary resistance and the transmitter and receiver are in a local series circuit. Assuming a potential drop between terminals c and b it is appar-V ent that the algebraic sum f the drop through the winding N2` and the drop drop through the receiver and transmitter, whence p K(n2"n1') (Ri'i'RIi Also since points d and e arefat the same potential, vit follows that the algebraic sum of thedrops through the transmitter and windings N1 and N3 is zero. Hence R1 "fs-n1 Ri 'ns-"x Also since the resultant ampere turns of the transformer windings equals zero l 314: (nz-'WOL from which wev get Y Il: n

"Now referringto equation 2 previously derived and remembering that during transmission 11=1s, said-equation may be written Y 1 AI a i Y K R1 zRA 'i' Ra a Y From equation (7) it is apparent that Y Y L nai'ni I Substituting this value in equation (2V) as z RFQS n') 11d-R, 11) Y Y s During reception it will be apparent from Fig. 4 that 11:12.V ySubstituting this value in equation (6),we get i Rx=yRa y Collecting formulrS, 9, 11 and 12, we have the following:V

R1=yRz Y Formul 13 may be written more gensaa .als

Y erally as f follows:

5 s or.. Na) (Na N...)

n Ned2 .Y Big Nac Nad I R1 Nad Nah The group of equations Adesignated A constltute the general design formulae for this type of substation and properly inter-V preted, as hereinafter set forth, determine the proper proportions of the elements of each substation herein disclosed. yFor the substation of Fig. 2, if we designate the winding ratios Y nl n:

"a d na Y by r and r', respectively, formulae 13 (or A) become R1 yRz `The third equation of the may be written preceding group R1 R3 e Tru-@P+R 'Combining YVthis equation with the first equation ofthe group, we have Furthermore combining the second and fourth equations of group 14, we have VSubstitutingthis value in equation. (15), we have Y (1 ,.)zilogryg` s 'y R4 which reduces to Solving equations (17), (18), and the first equation of group 14, we have Collecting, the design formulae of the substation of Fig. 1 may be expressed. as follows:

It will be observed that the above formulae determine the winding ratiosk and the resistances of the receiver and the auxiliary element in terms of the resistances of the line and the transmitter and the ratio y. Since in general these three values will be known, the remaining elements of the substation may be readily computed.

A modified form of substation is illustrated in Fig. 6. In this figure the receiver R and the winding N2 are seriallyT connected between the terminals b and c in parallel with the transmitter T. rlhe line L is serially connected with the winding N1 across the terminals band d in parallel with the winding N3. The auxiliary resistance X is connected across terminals d and e. Condensers may, if desired, be inserted as indicated to insure that the direct current supply will flow through the receiver It and transmitter T and to prevent its flow through the auxiliary resistance X. In this form of substation the current supply from the central station flows over the line and through windings N1 and N2, receiver It, transmitter T and winding N3 back to line.

The operation of the substation of Fig, 6 during transmission will be clear from Fig. 7 Since no current flows through the receiver, the auxiliary resistance and the transmitter are in a series circuit and the variable y potential set up by the transmitter causes a current Ilwhich is equal to I3 t o flow from the terminal ol through the auxiliary resistance and the transmitter to the point b where the current divides and a current I4 liows through the winding N1 and over the line L to point (l, while a current I-I4 flows through the winding N3 to the point d. Potentials are induced in the transformer windings whereby the terminal c is reduced to the same potential as terminal e, so that no current flows through the receiver during transmission.

'Ihe operation when receiving is illustrated in Fig. 8. An electromotive force applied to the line causes a current I4 to iow from the line terminal a through the winding N1, terminal b, winding N3, terminal d and back to the line. An induced current is set up in a local series circuit including the transmitter and the receiver so that a current I1 equal to I2 iiows from terminal b through the winding N2, through the receiver and the transmitter back to terminal I). Potentials are induced in the transformer windings whereby the terminal e is reduced to the same potential as the terminal (l, so that no current is wasted through the auxiliary resistance X. The only received energy which is wasted is that lost through the impedance of the transmitter T, which is no greater loss than would occur in a standard substation circuit.l

The design formul for the circuit of Fig. 6 may be readily determined from a consideration of formulae B. It will be observed in formulae B that R2 and R3 are independent of the transformer connections so that 100 the first two formulae of said group hold for all of the substations herein disclosed, since a comparison of Figs. 1, 5, 9, 13, 17 and 21 shows that the several substations didier from each other onlv in the trans- 105 former connections. Furthermore, if we write Nah for nl, Nm4 for n, and Nad for as, formulaj B become:

1 110 Rz-WQR 1 afl-yal C miran n U 5 Nrad .Nic-Nami n n Nad R4 1 120 Formulas C hold for all of the substations herein disclosed with a proper interpretation of the winding connections. A comparison of Figs. 2 and 6 shows that (taking 125 pif-oper account of the direction of winding) Hence the last twoV ofV formul .C become Writings?? as r and formulae for Fig. 6 are inFig.l 10 in which the receiver R and the, winding N2 are `connected 1n series .across the terminals a and e,- While the transmitter and A windlng N3 are connected in series across terminals 'd and ein parallel Vwith the auxiliary resistance The line L is connected across the terminals a and din parallel with the winding N1. Condensers may be inserted in series with the winding N1 and the-auxiliary resistance X, so that direct Acurrent from a distant source will How over the line and through winding N2, receiverv R, transmitter T and back to the line.r Y. v The operation in transmitting is illustrated in Fig. 11. Since no current Hows Y through the. receiver* R .the transmitter, auxiliary ,resistance and winding N3 are in .a local series circuit and the'actuation of the transmitter sets up an electromotive force whereby. a current I1 equal to the current I3 Howsrfrom terminal d through theV auxiliary resistance X, transmitter T and winding N3, back toA terminal d. 4 An induced currentI4 is thereby caused to How. from the terminal d through the winding N1 and over the line L back to the terminal d. Electromotive forces are induced ,inV the transformerwindingsof such value and direction that the terminal c is reduced to the same potential' as the terminal e and consequently noV current Hows'through the receiver during transmission.

j `In receiving, as shown' in Fig. 12, a'fcura rent I4 Hows over the line from Yterminal d to terminalV a. Since" no current Hows throughlthe auxiliary resistance the transmitter and Yreceiver are in a series circuit and a.v current I1 equal to the receiver current I2 Hows from terminal a, through the winding N2, receiver R, `transmitter T and winding N, torterminal d, ',.The current I1 winding N3 Y is greater than the current I4r and consequently a current 1,---14 Hows from terminal d to terminal a through the winding N1.,

f same potential as'terminal Yd so that no energy is wasted in theauxiliary resistance. Y Referring now to formulae C and compar ing Fig. 10 with Fig. 2, the number of turns of the transformer windings connecting ter-v minals a and b, a and c, and a and d may be expressed e Substitutin these valuesin formulae C the last two ormul become have as the design fornfulae for the3 substation of Fig. 10

Another modification is illustrated in Fig. 14 in whichrthe receiver R and Winding N2 are connected in series across terminals a and e, while the transmitter T and winding N1 are serially connected in parallel therewith. The auxiliary resistance X is serially connected with the winding N3 between terminal e and thejunction point b of the transmitter T and winding N1. The line L is connected between terminal a and the junction point d, between the auxiliaryresistance X and the winding N3. Condensers may be inserted in series with the winding N1 and the auxiliary resistance X,`so that direct current-,may How from the line to terminal a and thence in series through wind-VV ing N2, receiver R, transmitter T and VWind-V Y ing N3 back to the line.

The operation during transmission is indicated in Fig. 15.V Since no current Hows through the receiver,l the auxiliary resistance and the transmitterare in a series ciriary Vresistance Xl and the transmitter T to terminal Z). At the point Z) the current divides and a current I4 flows through the winding Nl and over the line L, back to terminal (Z, while a current L-L flows in parallel therewith from tl e terminal o, through the winding N:x to the terminal (Z. Potentials are induced in the transformer windings of such valuel and direction that the terminal c is reduced to the same potential as the terminal e,rv and consequently no current flows through the receiver during transmission.

rllhe action during reception is indicated in Fig. 1G. A potential applied to the line causes a current l@ to flow from the terminal 7) through the winding topoint Z and thence over the line te the terminal c. At terminal a the current divides and a eurrent l1 equal to L, flows through the winding N27 receiver' R and transmitter T to point while the current L-L liows in parallel therewith from the point Z through the winding N.L to the poiin. Z). lotentials are induced in the transformer which are of such value and direction as to reduce the terminal e to the same potential as the. terminal CZ, so'that no energy is wasted in the auxiliary resistance during the receiving operation. g

The design formulae for the substation of Fig. ll may be derived from the general design formulae C. Comparing Fi g. le with Fig. 2 it will be seen that the number of turns of the transformer' windings connecting terminals a and b. a and c and a and (Z may be expressed as follows:

Nah I nl N:le I -n2 Nad-:wilma Substituting these values in formulze C the last two formulre oi' said group become as r', the formulae appropriate to the subsiiationof Fig. 14 may be expressed as follows:

A lifth modification is illustrated. in Fig.

18 in which the transmitter T and winding N1 are serially connected between terminals a and e, the receiver B and winding Y2 are serially connected between terminals (Z and e, while the auxiliary resistance N. is connected in parallel therewith between terminals (Z and e. The terminals Z) and c of the transmitter and receiver, respectively, are connected by winding N.i which may have a condenser serially included in its circuit in order to properly direct the flow of current through the substation. The line L is connected to terminals a and tZ. Direct current from a distant source flows over the line to the terminal a and through the winding N1 and the transmitter 'l to the terminal e, where it divides and flows through the auxiliary resistance X in parallel with the receiver R and winding N2 to terminal (Z and thence back over the line.

The operation during transmission is indicated in Fig. 19. Since no current Hows through the receiver, the auxiliary resistance and transmitter are in a series circuit and a variable potential applied at the transmitter causes a current L equal to L to flow from terminal Z through the auxiliary resistance X and transmitter T to terminal b. A current I4 flows from terminal Z2 through the winding Nl and over the line L back to terminal CZ. A current idk-l1 liows from terminal Z through the winding N2 and through the winding N3 to terminal Z). Potentials are induced in the transformer windings of such value and direction that the terminal o is lowered to the same potential as terminal e, so that no current flows through the receiver.

The operation during reception is indicated in Fig. 20. A. potential applied to the line causes a current L to flow from terminal 0 through the winding N to terminal (Z and thence over the line L through winding N1 to point At point Z) the current divides and a current l1 equal to I2 flows from terminal Z) through the transmitter T and receiver R to terminal o, while a current L-ll flows in parallel therewith through the winding N3. The potentials induced in the transformer windings are of such value and direction as to reduce the terminal e to the same potential as terminal (Z, so that no energy is wasted in the auxiliary resistance during reception.

The design formulae for the substation of Fig. 18 may be determined from the general formulae C. Comparing Fig. 18 with Fig. 2, it will be clear that the number of turns connecting terminals a and Z9, a and 0, and a and cZ, respectively, may be expressed as follows:

Substituting these valuesl in formulae'C, the last two formulae of .the group becomey Hence .theV design formulae` for'the substa- 10 tion of Fig.v 18 may be expressed as follows:

' Fig. 22'illustrates. stil-lV another form of substation. VIn this .form the transmitterT t is connected in series withthewinding rN1 between terminals a and e, while the auxiliary resistance X is connected in series with the winding N3 between said terminals. The

receiver R is connected in series with the winding 2 betweenk terminals b; and e, while the lline L 1s connected between the terl potential and points Z and e are brought-to the same potential. Consequently no cur- Vrent flows Vfrom the point a through the minal aand the junction point d of the auxiliary resistance X andY winding N3. A Y condenser may be inserted in series with the n winding N3 so that direct current from v5 central station may flow overlth'e line toV terminal a, through the windingV N1 to point Z. -At point b the currei'itdivides, one path continuing through. the winding N2 and receiver R" to terminal e, whileithe other path filis through the transmitter T. From ter- 'ninal e the current flows through the auxilryresistance X to` the terminal cl and ,'b'abk over the line'. p .The

operation Vduring transmissionV is Y 4,5, IillustratedQin.,Fig.- 23. Since no current fiou'fsitliibiighthe receiver, thel auxiliary repoint a. AAt

i. g f current c. During'reception, as indicated in Figlvt, no currentv Hows Y througliwthgaiuxiliary* resistance. Consequentlynthgline is included .alga lina local closed gljithe winding .sistafnca transmitter and windingN1 are in K.- im .fz n. 'if

A heretoforeV in this specification.

understood therefore, that my invention isV ^`not limited to the specific embodiments herein illustratedgbut is broadly directed winding N,l to point and no current flowsV from the point c through the auxiliary resistance to point cl.

The design formulae for thevsubstation of Fig. 22 maybe determined from the general design formulae C. Comparing VFig.V 22 with Fig. 2,'the number of turns connecting terminals a and b, t and aand a and d, respectively, may be expressed as follows:

Nabzni i N6:n1'i77f2 atl-:n3 Y Substituting. these values in formulae G the last two equations of said group may be written y The design formulae for the substation of Fig. 22 may, therefore, be expressed as follows:

The type of substationl disclosed above ilo and lillustrated in the accompanying draw-k i ing is but one of a large number employing onlyone trans-former and one auxiliary lre-i sistance'and all` of these are ideal in the sense that they satisfy the fundamental requirem'entsfor an ideal substation, as stated It will be to providingr alsubstation comprising a element, which isiso. proportioned with reference to the line with whichlit is to be coopgnatiyely)fcoinbiiied,,that it is ideally effi- 4single transformer and only one auxiliaryV l naar, i 1e cient and substantially without side tone. Furthermore, I do noti'desire to lnnit the design of the substations herein disclosedv and illustrated to the accompanying design formulse. These formulzeare derived on the assumption that ideal transformers are employed and that the component elements` have no reactance, assumptions which are only approximately justified in practice. llVhen particularly high precision is desired I may, therefore, proportion the substation more precisely by taking into account the fact that the transformer impedances are finite, and that the line and the various substation elements may have in general some react-ance. The accompanying formulai however, give quite satisfactory results, and the methods by whichthey are derived will enable one skilled in the art,'to compute more precisely the substation constants` when desired.

lt will be understood that in the appended clainis, where certain elements are said to be conjugate, or certain impedance relations are said to exist, since in practice these conditions can, in general, only' be approximated, these expressions will be satisfied by structures substantially conforming thereto, especially where some compromise with regard to the rigid requirements is necessary in order to discriminate against line noise. It will also be understoodthat while I have specifically illustrated and described my invention as embodied in a telephone substation it is capable of many and varied embodiments which render it applicable in other kinds of signaling systems and consequently my invention is not to be limited to the particular form and use herein disclosed. It will be further understood that in this specification the word subs-tation is employed in its generic sense and that consequently its significance is not limited to a subscribers telephone station but embraces broadly a telephone station including a repeater station for relaying telephonic signals. Y

'What is claimed is:

l. A signaling circuit comprising a three winding induction coil and four component elements consisting of a path for the trans mission and reception of signals, a path including a transmitting apparatus. a path including a receiving apparatus, and au auxiliaryresistance; one of said three paths including a Ywinding Vof the induction coil, said first mentioned pathbeing shunted by at least a part of the windings of said induction coil, said auxiliary resistance being 'in a series circuit including said first mentioned path, and said auxiliary resistance and the other two paths having a common terminal.

2. A signaling circuit comprising a three winding induction coil and four component elements consisting of a path for the transmission and reception of signals, a path in. cluding a transmitting znpparatus, a path including a receiving apparatus, and an auxiliary resistance; one of said three paths iu- -luding a winding of the induction coil, said first mentioned path being shuntcd by at least a part of' the windings of said induction coil, said auxiliary resistance being in a series circuit including said first mentioned path` and said auxiliary resistance and the other two paths having a common terminal, said elements being so proportioned and re.- lated that the impedance of the combination comprising said induction coil, said first and last mentioned paths and said resistance, as seen from said second mentioned path, is equal to the impedance of said second inentioned path.V y

2l. A signaling circuit comprising a three winding induction coil and four component elements consisting of a path for the transmission and reception of signals, a path including a transmitting apparatus, a path including a receiving apparatus, and an auxilia ry resistance; one of said three paths including a winding of the induction coil, said first mentioned path being shuntcd by at least a part of the windings of said induction coil, said auxiliary resistance being in a series circuit including said first. mentioned path, and said auxiliary resistance and the other two paths having a common terminal, said elements being so proportioned and related that the impedance of the combination comprising said induction coil, said auxiliary resistance and said paths including transmitting and receiving apparatus, as seen from said first mentioned path, is equal to the impedance of said first mentioned path.

t. A signaling circuit comprising a three winding induction coil and four component elements consisting of a path for the transmission and reception of signals, a path including a transmitting apparatus, a path including a receiving apparatus, and an auxiliary resistance; one of said three paths including a winding of the induction coil, said first mentioned path being shuntcd by at least a part of' the windings of said induction coil, said auxiliary resistance being in a series circuit including said first mentioned path, and said auxiliary resistance and the other two paths having a common terminal,

said elements being so proportioned and related that said path including receiving apparatus and said path including transmitting apparatus are conjugate.

5. A signaling circuit comprising a three winding induction coil and four component elements consisting of a path for the transmission and reception of signals, a path including a transmitting apparatus, a path including a receiving apparatus, and an auxiliary resistance; one Vof said threeV paths including a Winding of the induction: coil, v said first mentioned path being shuntedby atleast a part of the windings of said induction coil, said auxiliary resistance being A`in a'series circuit including said first mentioned path, and said auxiliary resistance andthe other two paths having a common. l terminal, said element'sbeing so proportionedand related that said auxiliary resistance and said first mentioned path are coni ijugate.

6. VA signaling.` circuit Y comprising a three I `Winding induction coil and four component mission and reception of signals, a path ineleinents consisting of a path for the transcluding a transmitting apparatus, a path including a receiving apparatus, and an auxiliary resistance; one of said three paths including a Winding of the inductioncoil, said first mentioned path' being shunted by at least a part of the windings of said induction coil, said auxiliary i-esistancebeing in i a series ycircuit lincluding said first v,men-

, Copies Vot this patent may .be obtained for tionedpath, and said auxiliaryV resistance and the other two paths having a Acommon terminal, said elements being so proportioned and related that the impedance of the combination comprising said induction coil, saidirst and last mentioned paths and said resistance, as seen from Asaid second inentioned path, is equal to the impedance of said second mentioned path, the impedance of the combinationy comprising said induc` tion coil, said auxiliary resistance and said paths including transmitting andA receiving apparatus, as'seen'from said first mentioned' Y August 1917.

GEoRGE A. CAMPBELL.

five cents each, by addressing the Commissioner of Patents,

Washington, 1).'0. 

